For the process of vulcanization to be useful and successful, it should be controlled. It should begin when required, accelerate when needed and must stop at the right time. In the jargon of rubber technologists, these are termed as scorch resistance, acceleration and cure time, respectively. Scorch resistance is the time elapsed before vulcanization starts. It is necessary to have suitable scorch resistance so that there is enough time for mixing, storing and molding of the rubber mixture into the desired shape and size. Premature vulcanization results in the development of cracks in rubber, making the resulting products unusable. Once vulcanization begins, it should be completed as fast as possible in order to have practical batch cycle. Shorter cure times are preferred. Thus, it is important to control the way molecules interact with each other at different stages in order to achieve the desired physical property.
In order to achieve the above objective in the production and processing of rubber mixtures which contain vulcanizing agents, e.g., sulfur and accelerators, a certain amount of premature vulcanization, known as scorching, may occur before the proper vulcanization. The primary function of an accelerator or accelerator system is to increase the rate of the vulcanization process while allowing sufficient time to mix the accelerators into the rubber at an elevated temperature before vulcanization commences. This delay before the initiation of vulcanization is commonly referred to as scorch time. This scorching may occur, for example, in the mixer or during any of the subsequent processes such as extruding or calendaring. It is known that the risk of scorching can be reduced by the addition of a N-nitrosoamine, especially, N-nitroso-diphenylamine, to the rubber mixture. In certain circumstances, however, the use of these retardants leads to the formation of porous vulcanizates due to the nitroso group being split off. Further, these compounds not only influence the scorching but also have a marked effect on the entire vulcanization process, i.e., the vulcanization time is increased. Furthermore, a rather large dose of an N-nitrosoamine is required for achieving a given retarding effect.
It is recognized that many properties of a final rubber vulcanizate are important including stress strain properties and rheometer values. Other factors relating to the vulcanization which are of importance are the rate of cure, the cure time, the scorch behavior and the extent of cure. These physical properties can be altered either beneficially or detrimentally through the inclusion of chemical or components that impact upon the rate and state of vulcanization. Many accelerator combinations have been used in the rubber industry. Unfortunately, many of the known accelerators, such as morpholine containing accelerators, and dimethylamine containing accelerators yield volatile nitrosamines upon use. The use of accelerators which yield volatile nitrosamines have been significantly restricted in a number of countries, and the need to find a suitable replacement is ongoing.
U.S. Pat. No. 2,422,156 describes the process of reacting sulfur monochloride with tertiary alkylphenols to form tertiary alkylphenol sulfides for use in the vulcanization of rubber-like butadiene-1,3 polymerizates. Alkylphenol disulfides, specifically tertiary-amylphenol di- and poly-sulfides as well as tertiary-butylphenol di- and poly-sulfides have been available and used for many years as vulcanizing agents for rubber products. Typically the alkylphenol di- and poly-sulfides improve aging properties of the rubber as illustrated as formula (V) below.
                wherein                    R is an alkyl radical selected from the group consisting of primary, secondary and tertiary radicals; and            n is an integral value dependent upon the reactants used.                        
U.S. Pat. No. 3,812,192 also gives a process for the manufacture of polythiobisphenol. The process for making the p-cumylphenol disulfides involves the reaction of sulfur monochloride with the alkyl or alkylarylphenol with or without a solvent. Typical solvents are toluene, xylene, octane or other like solvents. The disulfides are illustrated below as formulas (VI) and (VII).
wherein                R1-4 are independently H, C1-4 alkyl, and halogen (preferably chlorine); and        n is an integral value from 2 to 3.        
U.S. Pat. No. 6,303,746B1 gives a process for making polymeric alkyl phenol sulfides that are useful as wood preservatives, the composition of which is illustrated below as formula (IV)
                wherein                    R is a lower alkyl containing from 1-4 carbons;            n is an integral value from 1 to 10; and            x is an integral value from 1 to 4; and            y is an integral value from 1 to 3.                        
Unlike the prior art, this invention involves alkyl-aryl phenol di- and polysulfides that show excellent cure times, scorch times, and heat aging properties are based on p-cumyl phenol and substituted derivatives thereof.